Why is Ujjain considered one of the seven holiest cities in Hinduism?

Ujjain is one of the Saptapuris — the seven cities in Hindu tradition that grant liberation (moksha) to those who visit them. It is home to the Mahakaleshwar Jyotirlinga, has ancient scriptural mentions in the Mahabharata and multiple Puranas, was the site of the Kumbh Mela (Simhastha), and has been a continuous centre of Shaiva worship, Vedic scholarship, and royal patronage for over three millennia.